This invention relates generally to computed tomography (CT) imaging and, more particularly, to reducing x-ray exposure in a multislice CT imaging system.
In at least one known CT imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
To reduce the total scan time required for multiple slices, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reducing scan time, helical scanning provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
To further reduce the total acquisition time, multi-slice CT has been introduced. In multi-slice CT, multiple rows of projection data are acquired simultaneously at any time instant. When combined with helical scan mode, the system generates a single helix of cone beam projection data. Similar to the single slice helical weighting scheme, the projection data can be “weighted” prior to filtered backprojection. Thus, one technical effect is the generation of a volumetric CT three-dimensional (3D) image of a scanned object.
Multislice CT systems are used to obtain data for an increased number of slices during a scan. Known multislice systems typically include detectors generally known as 3-D detectors. With such 3-D detectors, a plurality of detector elements form separate channels arranged in columns and rows. Each row of detectors forms a separate slice. For example, a two slice detector has two rows of detector elements, and a four slice detector has four rows of detector elements. During a multislice scan, multiple rows of detector cells are simultaneously impinged by the x-ray beam, and therefore data for several slices is obtained.
In known CT systems, the x-ray beam from the x-ray source is projected through a pre-patient collimating device, or collimator, that defines the x-ray beam profile in the patient axis, or z-axis. The collimator includes x-ray absorbing material with an aperture therein for restricting the x-ray beam. The process of restricting the x-ray beam to the desired fan beam profile is termed “collimation”.
With respect to restricting the x-ray beam, known collimators typically include two opposing metallic blades or eccentric cams that may be opened and closed to change the aperture width. The fan beam “thickness”, as measured along the z-axis, can be selected by adjusting the blade or cam orientation. The blades or cams also may be moved in a same direction to displace the centerline of the aperture. Changing the aperture centerline changes the fan beam angle with respect to the z-axis.
When performing a helical scan with a multislice CT scanner, detector rows spiral into the image region sequentially and the x-ray exposure to rows outside of the image region are not used. That is, a z-axis length of the beam exceeds the z-axis length of images that are produced. This unused exposure, or dose occurs at both the beginning and end of a helical scan, and the amount of helical overscan increases with the width of the multislice detector.